Improve error message

Prelude> class (A, B)

<interactive>:16:8: error:
    Unexpected type ‘A’
    In the class declaration for ‘GHC.Classes.(%,%)’
    A class declaration should have form
      class GHC.Classes.(%,%) a b where ...

in 7.10.2 it gave the more sensible

ghci> class (A, B)

<interactive>:30:7:
    Malformed head of type or class declaration: (A, B)

Trac metadata

Trac field	Value
Version	8.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

mentioned in issue #10362 (closed)

changed weight to 5

added Ttask Trac import labels

Trac metadata

Trac field	Value
Related	- → #10362 (closed)

Simon says "There should be no user-visible effects." in ticket #10362 (closed).

changed milestone to %8.0.1

changed weight to 7

Trac metadata

Trac field	Value
Version	8.1 → 8.0.1-rc1
TypeOfFailure	OtherFailure → IncorrectWarning
Priority	normal → high

added error messages label

If you had written

class T A b

Then the error would have been fine. What is special here? Just that it's using a type constructor that has built-in syntax.

I expect I could add a special case to dis-allow such class declarations, and I agree this one is strange because of the % signs. But is it really worth the trouble? Will people do this much?

closed

changed weight to 5

There wasn't really a consensus that special-casing this was worthwhile. Feel free to re-open if you disagree.

Trac metadata

Trac field	Value
Priority	high → normal
Resolution	Unresolved → ResolvedWon'tFix

I'm fine with closing it

added Pnormal label